1. Field of the Invention
The invention relates to a process for the purification of cyclohexanone which has been obtained by dehydrogenation of a cyclohexanone/cyclohexanol mixture, using hydrogen at elevated temperature and at reduced to elevated pressure.
Cyclohexanone which can be used in the process according to the invention can be obtained, for example, by first oxidising cyclohexane with oxygen in the presence of catalytically acting metal salts or in the presence of boric acid to obtain a reaction mixture which contains, inter alia, cyclohexanone, cyclohexanol and, possibly, its boric ester and cyclohexyl hydroperoxide. After thermal decomposition or decomposition by catalytic hydrogenation of the peroxides, and after separating off the constituents mentioned from the oxidation mixture, a cyclohexanone/cyclohexanol mixture is obtained. If cyclohexanone is the desired end product, this mixture, with or without preceding separation of cyclohexanone already present, is dehydrogenated, so that the cyclohexanol fraction is likewise converted into cyclohexanone. However, such a cyclohexanone still contains impurities in the range of 0.5 to 5 .permill., which are in part of unsaturated nature, but are also in part not completely known.
Furthermore, a cyclohexanone/cyclohexanol mixture can be obtained by hydrogenation of phenol, the cyclohexanol fraction of which mixture can likewise be dehydrogenated in the described manner to give cyclohexanone. Such a cyclohexanone also contains undesired accompanying substances, some unsaturated, in the range mentioned.
The impurities contained in such a cyclohexanone can only be partially eliminated, or not at all, even with great effort. However, they interfere with the following further processing of the cyclohexanone, for example in the subsequent preparation of caprolactam.
2. Description of the Related Art
Considerable efforts have already been made to separate off the impurities mentioned, with or without preceding isomerisation. Thus it has been disclosed to treat the cyclohexanone, obtained in the above manner, with alkali metal hydroxides, alkali metal alcoholates, phenolates or their solutions or with basic ion exchangers and then to rectify the mixtures, or to add auxiliary materials of the type mentioned during the rectification (German Auslegeschrift 1 188 584). However, the presence of compounds having alkaline reaction induces uncontrollable condensation reactions which not only lead to further impurities, but also represent considerable yield losses.
Since a portion of the interfering compounds are of unsaturated nature, attempts have already been made to convert these under hydrogenating conditions either into cyclohexanone or into compounds more easily separated off. This relates, for example, to the particularly interfering compounds cyclohexene and the various cyclohexenones. However, in all hitherto known hydrogenation processes, a large fraction of the cyclohexanone has always been further reduced to give cyclohexanol, for example according to BE 714 128, in which a catalytic gas-phase hydrogenation prior to the purification by distillation is disclosed; an examination showed that further by-products result here which must be additionally eliminated. It seemed, therefore, that a hydrogenating purification at various stages of the process for cyclohexanone preparation would remain unsuccessful. According to EP 411 455, therefore, an attempt was made to find a solution to the interfering effect of the impurities on the quality of caprolactam only on this product, that is by means of catalytic hydrogenation of an aqueous caprolactam solution.